Karen is the first sister that Wataru meets on Promised Island and is considered the heroine of the story, since she seems to speak for all of them with how much they love and care about their big brother. Knows how to play piano, is sweet, gentle, and is your all around good girl. In the anime when Wataru first arrives and almost drowns in the ocean, she comes to his rescue, and soon meet again when he hears her play the piano at school.

In Sister Princess RePure, she adores her older brother as her hero, when he makes a dress that he gave her as a present more stunning that it originally was. It's been said that out of all of the sisters, Karen is the one who seems to have the strongest heartfelt and emotional connection with her brother. Supposedly, Karen is the only sister who is fully related to Wataru.